Advances in consumer electronics are creating as need for high speed analog-to-digital converters in applications such as high definition television (HDTV), magnetic recording sampling detectors, medical imaging, and digital transmission links for telecommunications and cable networks. Many of these applications are implemented in CMOS (complementary metal oxide semiconductor) rather than the more expensive BiCMOS technology.
Flash analog-to-digital converters (ADC) have been employed to realize very high speed conversions. The analog input voltage is fed to 2.sup.N -1 comparators in parallel which are coupled to a resistor ladder producing a predetermined number of ascending reference voltages. The comparators generate a cyclic thermometer code according to the input voltage level as compared with the reference voltage levels. The cyclic thermometer code is then decoded to produce the digital output. Flash analog-to-digital converters are fast but require a large number of comparators which typically take up a large area and are generally power hungry.
Accordingly, techniques have been sought to reduce the number of comparators needed in a flash analog-to-digital converter. Folding is an analog preprocessing step used to achieve this end. The number of comparators required is reduced by the degree of folding. Folders used to generate folded signals are typically implemented with cross-coupled differential pairs. Interpolating is another technique that may be combined with folding to generate intermediate folded signals to reduce the number of folders required to generate the same number of folded signals.
As described in M. P. Flynn et al., CMOS Folding A/D Converters with Current-Mode Interpolation, IEEE Journal of Solid-State Circuits, Vol. 31, No. 9, September 1996 (hereinafter "Flynn et al."), differential pair-based folder circuits may be used to generate folded current signals, which are then interpolated to generate intermediate folded current signals by a current divider circuit. Flynn et al. is incorporated herein by reference. The current signals are then provided to comparators to produce the cyclic thermometer code. Past comparators operate to compare voltage levels from the folding and interpolating circuits in analog-to-digital converters, not current levels. Accordingly, a current mode comparator capable of very fast sampling speeds is desired. Further, it is desirable to provide a comparator based on standard CMOS technology which is compatible with the technology of choice for many applications.